It has long been known, that when people walk, jog, or run, a significant percentage of their forward and upward kinetic energy is wasted and lost. This loss results in two undesirable effects, the first of which is locomotion inefficiency. More specifically, a person's potential for attaining their maximum walking/running speed and endurance as well as jumping height (without motorized assistance) is diminished. The second negative effect of this lost energy is manifested in the substantial shock which is imparted to a person's knees and feet when impacting with the ground while running or jumping. As a result, great effort has been exerted by both independent inventors and large corporations to develop effective “energy-return” footwear that could replace standard athletic footwear.
Energy-return footwear designs, generically referred to as “spring-shoes”, have been around for centuries and may be as old as the invention of springs themselves. The concept is obvious: build shoes with springs or some other energy storage device and augment a person's performance and/or comfort. However, this has been a difficult task as evidenced by the hundreds of such patents, filed since the mid 1800s, with very few designs being accepted in the marketplace.
Designing an effective energy-return shoe requires identifying and meeting several important objectives. The shoe must: 1) store and return a significant portion of kinetic energy, 2) be stable and controllable, 3) promote a natural motion during locomotion, 4) be both durable and reasonably light, 5) be simple in design, and 6) be designed with spring geometry that can be optimized for either comfort or performance or any compromise in between. Creating a shoe that successfully combines these qualities would represent a revolutionary advancement in the art and insure its widespread acceptance by consumers.
In order to store and return a significant portion of energy during locomotion (i.e. the first objective), a shoe's sole must transfer kinetic energy due to heel compression forces, and return them to the toe, during liftoff. That is, the heel and toe portions of the soles must work together upon heel-strike and toe-lift, allowing greater energy storage and return. Additionally, the sole must be both substantially compressible and free to compress and expand without hindrance (i.e. not be dampened by the walls of a rubber sole or any other impediments). Furthermore, the spring rates should be tailored to the user's weight and specific use such that the springs store and return as much of the impact forces as possible. These qualities work together to insure that during toe-off the wearer will experience the right force at the right time for a reasonable duration. Energy-return can be even further augmented if a shoe's sole can be held in the compressed position through the point of peak load and released during toe lift-off. Such an arrangement would allow for spring rates 2 to 3 times higher than would otherwise be used.
The second objective of an effective energy-return footwear design is that it be both stable and controllable. This aspect is important both for allowing a user to effectively use the energy that is returned and for obvious safety reasons. Shoes with compressible soles that have been designed with an emphasis on energy-return have struggled in meeting this objective. This is often due to the fact that the lower sole is not constrained in its movement relative to the upper sole and there is no provision for the use of a wearer's toes (or a structure that performs in a similar function) or in the case of higher compression designs there is a lack of ankle support. More specifically, the lower sole may slide or skew longitudinally or laterally, or sometimes in any direction, relative to the wearer's foot and the design may employ a rigid upper and lower sole that does not bend at the ball of the foot limiting the user's balance and traction that toes can provide. In many cases, where sole designs have sought to address these limitations, they have relied on the use polymers instead of, or in addition to, mechanical devices or they have limited the use of mechanical devices to the heel region. In so doing, these designs have compromised energy-return.
The third objective of an effective energy-return shoe is that the sole design promotes a natural motion during locomotion. This is important because energy-return footwear that encourages unnatural motions by the wearer compromises the benefits of storing and returning energy in locomotion and may also pose a safety risk. To provide for natural movement, the shoe sole design must: provide for the effective use of the wearer's toes (i.e., upper and lower toe sole pivoting from an upper and lower heel sole), release the stored-energy in a direction that is perpendicular to the user's foot (i.e. generally in line with the wearer's leg), provide a rigid lower sole frame with a flexible tread surround that is likened to a bare foot (or in the case of a higher-compression design, a laterally tilting lower sole with longitudinally pivoting heel and toe pads) and release the stored energy at an optimal time during the stride. Other energy-return footwear designs that have inadequately addressed these requirements have failed to promote a natural running motion and would not be considered a viable alternative to standard athletic footwear.
The fourth objective of an effective energy-return footwear design is that it be both durable and reasonably light. This goal represents a significant challenge for full-length mechanical soles due to the extreme forces at play and fact that they usually rely on metal components that are either reasonably light or durable but not both. Although major advancements have been made in the area of materials engineering (i.e. composite fibers) these developments alone cannot solve this problem. The solution, instead, is found in designing an efficient mechanical system that employs structure-leverage and the efficient use of materials. For example it is preferred that a large percentage of the sole's height or thickness be compressible (i.e. that it is not unnecessarily tall.)
The fifth characteristic of an effective energy-return shoe is that it be simple in design. This is as important for energy-return footwear designs as it is for most any mechanical design. Benefits to design simplicity include reduced friction, improved durability and minimized manufacturing cost.
The sixth objective of an effective energy-return shoe is that it be designed such that the spring geometry can be optimized for either comfort or performance or any compromise in between. There exist many energy-return footwear patents that recognize the benefit of tailoring the energy-storage component's capacity to a user's weight and/or type of activity, but the vast majority of these designs do not address the merits of managing the force rates by which energy is stored and returned. The underlying premise of this concept is that there is a tradeoff between energy-absorption and energy-return. That is, a shoe that is designed for comfort would not be ideally suited for performance applications and vice-versa. More specifically, the energy-return forces for a comfort-designed shoe should be linear and progressive (for example as delivered by a simple compression spring as widely exemplified in the prior art). On the other hand, energy-return forces for a performance shoe should be either constant or regressive. For example, employing a regressive force rate would mean that as the shoe compresses, the resistance force diminishes and conversely, as the shoe expands, the expansion force increases. Additionally, the force curve could be developed as a wide range of compromises between pure comfort and pure performance. Such variety of force rate characteristics are achieved by using compression springs, torsion springs or extension springs between two opposing hinges or a spring combination thereof. The method and structure for creating force rate curves optimized for a variety of applications and preferences will be explained in the Detailed Description of the Invention section.
These six objectives represent therefore the ideal characteristics that have eluded spring-shoe designers for years. Certain designs may have excelled in one or two or three of these areas but none has combined all objectives in a single package. The following examples are provided to illustrate the limitations of these prior designs.
A patent of interest is U.S. Pat. No. 4,133,086 “Pneumatic Springing Shoe” to Brennan which discloses a rigid lower sole supporting an upper sole via two pneumatic springs. This design is limited by lack heel-to-toe energy transfer and an inflexible lower sole which prevents a natural running motion. Also this design is unnecessarily heavy and bulky due to the fact that it requires a tall sole to produce the desired amount of compression.
U.S. Pat. No. 4,196,903 “Jog-Springs” to Illustrato employs a full-length spring-suspended sole but does not provide a correlation between the heel springs and the toe springs to effectively transfer energy from heel to toe. Additionally, it is limited by its inherent instability and uncontrollability and unnatural use.
U.S. Pat. No. 4,912,859 “Spring-shoe” to Ritts discloses a full-length mechanical sole that relies on a hefty longitudinal link to resist lateral tilting. This design is limited by a lack of heel-to-toe energy transfer and inflexible lower sole which prevents a natural running motion. Also this design relies on the stoutness of this link to limit such movement and thus adds considerable weight to the sole.
Another patent of interest is U.S. Pat. No. 4,936,030 to Rennex titled, “Energy Efficient Running Shoe.” This patent recognizes that an increase in performance requires transfer of energy from heel-strike to the ball or toe region during step-off via a series of complex levers and shafts. This patent recognizes that an increase in performance may be possible with a system to hold the energy loaded during heel-strike and release it from the ball or toe region during step-off. This design employs a ratchet to hold the loaded spring and triggers its release by bending the toe section of the shoe. These structures provide neither an optimum nor precise timing for energy release. The optimum timing of energy release is immediately following ball peak-force during step-off. The system releases the loaded spring either: 1) when said spring reaches a certain and fixed degree of compression, 2) when said spring reaches the limit of compression during push-off, or 3) after a fixed time delay. Although the patent neither explains nor diagrams the process by which it accomplishes (2) or (3), these methods are inadequate and not optimal. The first and third processes are based on fixed criteria and cannot adapt to the variable forces and time periods during normal running. The second process is inadequate because it releases the spring prematurely. A user, during a turn or stop may load the forces on his forefoot at constant level before he has picked his final direction. This process therefore, can cause the user to lose control. The system does not guarantee nor does it disclose that the ball and heel will compress in a parallel manner. Additionally, these complex structures fall short in the area of promoting natural movement; provide a platform for stability, durability and lightness.
U.S. Pat. No. 5,343,637 “Shoe and Elastic Sole Insert Therefore” to Schindler discloses two elastic inserts contained within a hollow and flexible rubber sole. Although this design does allow flexibility at the ball of the foot, the lack of a framework for the lower sole results in an uncontrolled compression and expansion of the spring. This limits the user's ability to balance and move in a controlled fashion. To the extent that stiffer sole walls are used to improve stability, there is a commensurate increase in damping which diminishes the energy-return capacity of the spring.
Another patent of interest is U.S. Pat. No. 5,343,639 “Shoe with an Improved Midsole” to Kilgore et al., employs a “plurality of compliant elastomeric support elements” in the heel to absorb impact forces. Although this design attempts to make advances in the resilient material employed, it is still limited in the same way that all polymer-based designs are limited. More specifically, this design is compromised by the fact that there is no provision for the transfer of heel impact forces to the toe during lift-off, the sole is not substantially compressible and there is no provision for optimizing the energy-return force curves for performance applications.
In another patent of interest, U.S. Pat. No. 5,435,079 “Spring Athletic Shoe” to Gallegos discloses a conical heel spring. This design is limited by the lack of energy transfer from the heel to the toe. Additionally this design is limited in that the spring geometry cannot be tailored to anything other than comfort (i.e. not for performance applications).
U.S. Pat. No. 5,517,769, “Spring-Loaded Snap-Type Shoe,” to Zhao. This patent recognizes that a significant increase in performance may be possible with a system to hold the energy loaded during heel-strike and release the energy during step-off. The disclosed system used a ratchet to hold the loaded spring and triggers its release by bending the toe section of the shoe. Thus, this system attempts to time the release of energy during step-off. This system provides neither an optimum nor precise timing for energy release. The optimum timing of energy release occurs immediately following the decrease force during step-off. The system releases the loaded spring when the user bends at the ball of the foot which is not necessarily during and perhaps never at the optimum time. The system also returns energy to the heel alone. This is not ideal because the heel is not in contact with the ground during step-off. The system also requires a hollow cavity extending the length of the foot for the containment of the ratchet and spring system but does not provide a suspension system for maintaining this cavity leaving it to compress randomly.
Another patent of interest is U.S. Pat. No. 6,029,374 to Herr: “Shoe and Foot Prosthesis with Bending Beam Spring Structures.” This patent attempts to address the problem with carbon fiber bending beam springs. This patent also attempts to address the need for both heel and toe springs that prevent lateral movement. This structure is inadequate for some of the following reasons: 1) It does not provide a strictly parallel postured upper and lower sole and thus it cannot return more than half the user's weight, 2) it does not provide a parallel upper and lower toe sole and therefore depends on a tapered leaf spring for traction and control in which it does not provide either in an optimum way, 3) it does not provide a hold and release system (HRS) that limits the combined load forces of the springs to approximately the user's weight.
Another patent of interest is U.S. Pat. No. 6,282,814 B1 “Spring Cushioned Shoe” to Krafsur, et al., wherein wave springs are placed in the heel and toe regions of a polymer sole. Although this sole design does include mechanical components (i.e. wave springs) in both the toe and heel regions of the sole, their effectiveness is greatly diminished by their independence and disconnection which prevents a transfer of energy from the heel to the toe. Also, they are limited by the dampening effect of the polymer sole in which they are placed. Additionally, wave springs themselves tend to lack free movement due to the friction generated by their “crest to crest” design.
Another patent of interest is U.S. Pat. No. 6,684,531 to Rennex for a “Spring Space Shoe,” which is hereby incorporated by reference. This patent introduces a spring-lever mechanism that provides some level of energy absorption upon impact and energy-return during step-off and discloses a series of linkages that prevent longitudinal tilting between the top and bottom soles. This design, however, is limited in its stability and controllability because it lacks a means to prevent front-to-back sliding of the user's foot with respect to the lower sole of the shoe. For example, in the mechanism of FIG. 1a, there is nothing to prevent the right side (heel of foot) of the mechanism from moving forward with respect to the left side (ball of foot). Additionally, the structures disclosed are not designed to prevent any substantial lateral forces from causing the upper sole to slide sideways relative to the lower sole. Another limitation in this design is that it does not include a toe sole structure, thereby eliminating the balance and control and traction that toes provide to a person. Furthermore, the disclosed “heel hugger” structure does not provide for an energy-return vector, perpendicular to the user's foot. This means that the energy is not released in a direction that is in-line with the force of the user's leg. Additionally it does not either provide a flexible tread/sole around the perimeter of the lower sole nor does it disclose a longitudinally non-tilting yet laterally pivoting lower sole with longitudinally pivoting heel and toe pads, so a user's lateral movement is constrained and becomes awkward. Finally, although it does suggest that a combination of different springs may be used to manage spring forces, it does not disclose how a torsion spring could be included for this purpose and how it could be used to effectively include it in the structure.
Another patent of interest is U.S. Pat. No. 6,719,671 B1 “Device for Helping a Person to Walk” to Bock. This patent discloses a large leaf spring that extends from the back of the knee to the shoe sole as a means of storing and releasing energy during locomotion. Although this design affords a large degree of sole compression, it also weighs more than 5 times the amount of other energy-return footwear. This is due, in large part, to the design and therefore size of the leaf spring. Additionally, this patent does not provide a strictly parallel postured upper and lower sole of normal length nor does it provide a parallel upper and lower toe sole and therefore does not provide adequate balance and control. Furthermore, it does not provide a longitudinally pivoting lower sole and therefore does not allow for adequate traction agility and control.
Finally, U.S. Pat. Application 2004/0177531 titled, “Intellegent Footware System,” discloses a spring heel that adjusts tension in response to impact forces to modify performance characteristics. Although, this design accounts for the stiffness requirement of a spring depending on the activity it is limited in a number of respects. First there is no transfer of energy from the heel to the toe. Additionally the spring geometry can not be altered and so the shoe is only optimized for comfort and would not be very effective in performance applications. Also, like other shoes that have a polymer component, this design is compromised in its ability to freely store and return energy.
Spring-shoes thus have not been entirely satisfactory in that they have not permitted users to concurrently experience substantial energy-return, traction, control, safety and agility, and therefore have been viewed as incomparable and inferior to non-spring-loaded footwear. Furthermore, we are no closer to reaching the dream of augmenting performance, as no non-fuel-propelled footwear device has so far allowed users to increase their maximum running speed. (While some have allowed an increase in stride-length, their unnatural use and/or excessive weight prevent users from running any faster than with standard running shoes.). Additionally, these prior efforts have employed either very complex, expensive and unreliable structures and/or ineffective and imprecise structures. What is needed is a shoe system that achieves the aforementioned six objectives.